Radio access system

ABSTRACT

A wireless access system is provided which prevents the occurrence of a hidden terminal problem and minimizes the reduction in throughput. In the wireless access system, an access point ( 12 ) divides terminals into groups such that terminals in one group cannot recognize radio waves sent from terminals in another group, and performs communication with the terminals on a per-group basis. By this, the wireless access system avoids the occurrence of the hidden terminal problem. RTS/CTS packets used to avoid the occurrence of the hidden terminal problem are exchanged between the access point ( 12 ) and terminals ( 131  to  133, 141  to  142 ) on a per-group basis, whereby overhead is reduced and the reduction in throughput is minimized.

TECHNICAL FIELD

The present invention relates to a wireless access system used for awireless LAN system and the like, and more particularly to a wirelessaccess system which prevents the occurrence of a hidden terminal problemand minimizes the reduction in throughput.

BACKGROUND ART

FIG. 11 is a diagram for explaining a hidden terminal problem in aconventional wireless LAN system. In FIG. 11, the conventional wirelessLAN system includes a wireless LAN access point 920 and a plurality ofterminals 930 to 932. The conventional wireless LAN system employs aCSMA/CA scheme to prevent the collision of data transmitted betweenterminals. In such a wireless LAN system, however, if an obstacle 940which blocks radio waves is present between the terminals, the terminal932 cannot receive radio waves transmitted from the terminals 930 and931. Furthermore, the terminals 930 and 931 cannot receive a radio wavetransmitted from the terminal 932. Therefore, as viewed from theterminals 930 and 931, the terminal 932 is a hidden terminal. As viewedfrom the terminal 932, the terminals 930 and 931 are hidden terminals.That is, the so-called hidden terminal problem occurs in the wirelessLAN system.

As one of the methods of solving the hidden terminal problem, there isan RTS/CTS control scheme. FIG. 12 is a diagram for explaining theoperation of the RTS/CTS control scheme. In FIG. 12, a terminal 930transmits, when beginning data transmission, an RTS packet 901 as atransmission request to an access point 920. Once the access point 920has received the RTS packet 901, the access point 920 responds with aCTS packet 902 as a transmission permission to the terminal 930. Oncethe terminal 930 has received the CTS packet 902, the terminal 930begins transmission of a data packet 903. The CTS packet 902 is alsoreceived by terminals 931 and 932. Once the terminals 931 and 932 havereceived the CTS packet 902, the terminals 931 and 932 go into atransmission standby state while the terminal 930 is transmitting thedata packet 903. As described above, in a wireless LAN system employingthe RTS/CTS control scheme, while a certain terminal is transmittingdata, other terminals are prohibited from transmitting data. By doingso, the collision of transmitted data can be avoided between terminalswhich cannot receive from each other radio waves transmitted therefrom,and accordingly, the occurrence of the hidden terminal problem can beprevented.

Note, however, that in the wireless LAN system employing the RTS/CTScontrol scheme, each time data communication is performed between theaccess point 920 and the terminals 930 to 932, RTS/CTS packets need tobe transmitted and received. Because of this, the time consumed byoverhead other than data communication is increased, causing a reductionin throughput. As a system for preventing such a throughput reduction,conventionally, a wireless LAN system is disclosed which switches thecommunication mode depending on whether or not a hidden terminal ispresent among a plurality of terminals (for example, Patent Document 1).

A conventional wireless LAN system which switches the communication modeperforms data communication between an access point and terminals usingthe RTS/CTS control scheme when a hidden terminal is present, or usingthe CSMA/CA scheme when there is no hidden terminal. This eliminates theneed to transmit and receive RTS/CTS packets when there is no hiddenterminal, making it possible to prevent a reduction in throughput.

[Patent Document 1] Japanese Laid-Open Patent Publication No.2002-217913

DISCLOSURE OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

In the conventional wireless LAN system which switches the communicationmode, however, when a hidden terminal is present, each time datacommunication is performed between the access point 920 and theterminals 930 to 932, RTS/CTS packets need to be transmitted andreceived. Hence, when a hidden terminal is present, the time consumed byoverhead is increased after all, causing a reduction in throughput.

Therefore, an object of the present invention is to provide a wirelessaccess system which prevents the occurrence of the hidden terminalproblem even when terminals which cannot receive from each other radiowaves transmitted therefrom are present, and which minimizes thereduction in throughput.

The present invention is directed to a wireless access system forperforming transmission and reception of data using a radio wave. Inorder to attain the object mentioned above, a wireless access system ofthe present invention comprises a plurality of terminals and an accesspoint. The plurality of terminals each perform transmission andreception of a radio wave in accordance with a predeterminedinstruction. The access point divides the plurality of terminals intogroups so as to create a plurality of groups of terminals, and providesthe predetermined instruction to give a communication permission, theplurality of groups each having terminals which can receive from eachother radio waves transmitted therefrom, the communication permissionallowing only one of the plurality of groups to communicate with theaccess point.

Preferably, the wireless access system further comprises at least oneremote antenna which performs, in a location away from the access point,transmission and reception of a radio wave with any of the plurality ofterminals; and a transmission line which connects between the remoteantenna and the access point.

The transmission line may be an optical transmission line through whichan optical signal is transmitted. In the case where the transmissionline is an optical transmission line, the wireless access system mayfurther comprise an electrical-optical/optical-electrical conversionsection and an optical-electrical/electrical-optical conversion section.The electrical-optical/optical-electrical conversion section converts anelectrical signal inputted via the access point into an optical signaland outputting the optical signal to the optical transmission line, andconverts an optical signal inputted via the optical transmission lineinto an electrical signal and outputting the electrical signal to theaccess point. The optical-electrical/electrical-optical conversionsection converts an optical signal inputted via the optical transmissionline into an electrical signal and outputs the electrical signal to theremote antenna, and converts an electrical signal inputted via theremote antenna into an optical signal and outputs the optical signal tothe optical transmission line. An optical fiber cable may be used forthe optical transmission line. Further, free space may be used for thetransmission line.

Preferably, the access point may provide, as the predeterminedinstruction, to the plurality of terminals specific information aboutand a communication permitted time for terminals which are included in agroup to which the communication permission is given.

The access point may include a first group creation section. The firstgroup creation section transmits a response request packet to each ofthe plurality of terminals in any order, receives, as a response to theresponse request packet, a response packet from each of the terminals,and then creates the plurality of groups based on specific informationset in each of the received response packets.

In addition, the terminals each may include a second group creationsection. The second group creation section sets, in the response packet,specific information about its own terminal and specific informationreceived from other terminals, and responds to the access point with theresponse packet as a response to the response request packet receivedfrom the access point.

The access point may further include a first communication controlsection. The first communication control section sets, in a case wherethe access point receives from any of the plurality of terminals an RTSpacket as a transmission request, specific information about and acommunication permitted time for terminals which are included in a groupfrom which the RTS packet has been transmitted, in a CTS packet as aninstruction to give the communication permission, and transmits the CTSpacket to the plurality of terminals.

The terminals each may further include a second communication controlsection. The second communication control section transmits, in a casewhere data to be transmitted is generated, the RTS packet to the accesspoint, receives from the access point the CTS packet as a response tothe RTS packet, and determines, if the specific information set in thereceived CTS packet contains specific information about its ownterminal, that the communication permission is given to its ownterminal, and performs communication with the access point for a periodof the communication permitted time.

The access point may set a predetermined time for the communicationpermitted time. The access point may determine the communicationpermitted time in accordance with an instruction from an operator, oraccording to a number of terminals included in the plurality of groups.Alternatively, the access point may determine the communicationpermitted time according to an amount of transmission standby data heldin the terminals included in the plurality of groups.

The present invention is also directed to a wireless access method ofperforming transmission and reception of data using a radio wave. Inorder to achieve the object mentioned above, a wireless access method ofthe present invention provides the following steps to an access pointand a plurality of terminals. The access point comprises the steps of:transmitting a response request packet to the plurality of terminals inany order; receiving, as a response to the response request packet, aresponse packet from each of the terminals; and creating the pluralityof groups based on specific information set in each of the receivedresponse packets. The terminals each comprise the steps of: receivingthe response request packet from the access point; and setting, in theresponse packet, specific information about its own terminal andspecific information received from other terminals, and responding tothe access point with the response packet as a response to the responserequest packet.

The access point may further comprise the steps of: setting, in a casewhere the access point receives from any of the plurality of terminalsan RTS packet as a transmission request, specific information about anda communication permitted time for terminals which are included in agroup from which the RTS packet has been transmitted, in a CTS packet asan instruction to give the communication permission; and transmittingthe CTS packet to the plurality of terminals. The terminals each mayfurther comprise the steps of: transmitting, in a case where data to betransmitted is generated, the RTS packet to the access point; receivingfrom the access point the CTS packet as a response to the RTS packet;and determining, if the specific information set in the received CTSpacket contains specific information about its own terminal, that thecommunication permission is given to its own terminal, and performingcommunication with the access point for a period of the communicationpermitted time.

EFFECT OF THE INVENTION

As described above, according to the present invention, the groupcreation section divides a plurality of terminals into groups so as tocreate groups of terminals, each group having terminals which canreceive from each other radio waves transmitted therefrom. Then, thecommunication control section assigns a communication interval and astandby interval individually to each group, and controls communicationbetween the access point and the terminals to be performed on aper-group basis. By doing so, the occurrence of the hidden terminalproblem can be prevented between terminals which cannot receive fromeach other radio waves transmitted therefrom. Also, since RTS/CTSpackets are transmitted and received on a per-group basis, the timeconsumed by overhead is reduced as compared with a method oftransmitting and receiving RTS/CTS packets on a per-terminal basis,which is one of the conventional methods of solving the hidden terminalproblem; accordingly, the reduction in throughput can be minimized.

BRIEF DESCRIPTION ON THE DRAWINGS

FIG. 1 is a block diagram showing an exemplary network configuration ofa wireless access system according to a first embodiment of the presentinvention.

FIG. 2 is a diagram schematically showing a state in which communicationis performed between an access point 12 and each group.

FIG. 3 is a block diagram showing an exemplary functional configurationof the wireless access system according to the first embodiment of thepresent invention.

FIG. 4 is a diagram for explaining the operation of a group creationsection according to the first embodiment.

FIG. 5 is a diagram for explaining the operation of a communicationcontrol section according to the first embodiment.

FIG. 6 is a diagram for explaining the operation of a communicationcontrol section according to a second embodiment.

FIG. 7 is a diagram for explaining the operation of a wireless accesssystem which transmits, during a communication interval, the length ofdata generated during a preceding standby interval.

FIG. 8 is a diagram showing an exemplary network configuration of awireless access system according to a third embodiment.

FIG. 9 is a diagram showing an exemplary network configuration of awireless access system according to a fourth embodiment.

FIG. 10 is a diagram showing an exemplary network configuration of awireless access system which performs switching between a plurality ofaccess points.

FIG. 11 is a diagram for explaining a hidden terminal problem in aconventional wireless LAN system.

FIG. 12 is a diagram for explaining the operation of an RTS/CTS controlscheme.

DESCRIPTION OF THE REFERENCE CHARACTERS

10 COMMUNICATION NETWORK

11 COMMUNICATION LINE

12, 20, 31, 32 ACCESS POINT

12 g, 13 g GROUP CREATION SECTION

12 c, 13 c COMMUNICATION CONTROL SECTION

15 RADIO-WAVE SHIELD

21, 22 REMOTE ANTENNA

23 TRANSMISSION LINE

24, 28, 29, 34, 37 OPTICAL-ELECTRICAL/ELECTRRICAL-OPTICAL CONVERTER

25, 26 REMOTE ANTENNA UNIT

27 OPTICAL TRANSMISSION LINE

33 SWITCH

131 to 133, 141 to 142 TERMINAL

P131 to P133, P141 to P142 PACKET

221 to 228 PACKET

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

FIG. 1 is a diagram showing an exemplary network configuration of awireless access system according to a first embodiment of the presentinvention. In FIG. 1, the wireless access system according to the firstembodiment includes an access point 12 and a plurality of terminals 131to 133 and 141 to 142. The access point 12 is connected to acommunication network 10, such as an Internet network, through acommunication cable 11.

The access point 12 converts an electrical signal inputted through thecommunication cable 11 into a radio signal and then transmits the radiosignal to free space, and also converts a radio signal received from theterminals 131 to 133 and 141 to 142 into an electrical signal and thentransmits the electrical signal to the communication cable 11. Theterminals 131 to 133 and 141 to 142 each demodulate a radio signalreceived from the access point 12 and thereby obtain data, and alsotransmit, as a radio wave, a radio signal obtained by modulating data,to free space.

Now, the case is considered where a radio-wave shield 15 which blocks aradio wave is present between the terminals 131 to 133 and the terminals141 to 142. In this case, the terminals 141 to 142 cannot receive radiowaves transmitted from the terminals 131 to 133 because of the presenceof the radio-wave shield 15. Similarly, the terminals 131 to 133 cannotreceive radio waves transmitted from the terminals 141 to 142. That is,as viewed from the terminals 131 to 133, the terminals 141 to 142 arehidden terminals. As viewed from the terminals 141 to 142, the terminals131 to 133 are hidden terminals.

The operation of the wireless access system according to the firstembodiment of the present invention will be described below. In thewireless access system, first, the access point 12 creates a pluralityof groups each having no hidden terminal, by grouping together terminalswhich can receive from each other radio waves transmitted therefrom. Inthis example, the access point 12 groups the terminals 131 to 133 asgroup A and the terminals 141 to 142 as group B.

Next, the access point 12 assigns each group with a communicationinterval and a standby interval, and performs communication with theterminals on a per-group basis. FIG. 2 is a diagram schematicallyshowing a state in which communication is performed between the accesspoint 12 and each group. In FIG. 2, while the group A is performingcommunication with the access point 12, the group B stands by forcommunication with the access point 12. Similarly, while the group B isperforming communication with the access point 12, the group A stands byfor communication with the access point 12. This avoids the collision ofradio waves transmitted between the groups A and B, and accordingly, theoccurrence of the hidden terminal problem can be prevented.

The access point 12 determines the communication interval and thestandby interval to be assigned to each group, according to the numberof terminals belonging to each group. For example, in the case wherethere are three terminals belonging to the group A and two terminalsbelonging to the group B, the access point 12 can assign the group Awith a three-second communication interval and the group B with atwo-second communication interval.

FIG. 3 is a block diagram showing an exemplary functional configurationof the wireless access system according to the first embodiment of thepresent invention. FIG. 3(a) is a diagram showing a configuration of theaccess point 12. FIG. 2(b) is a diagram showing a configuration of theterminal. In FIG. 2, the access point 12 includes a first group creationsection 12 g and a first communication control section 12 c. Theterminal includes a second group creation section 13 g and a secondcommunication control section 13 c.

In the wireless access system according to the first embodiment, thefirst group creation section 12 g of the access point 12 and the secondgroup creation section 13 g of the terminal cooperatively create theaforementioned groups. Thus, a configuration in which the first groupcreation section 12 g and the second group creation section 13 g arecombined together can also be simply taken as a group creation section.Also, the first communication control section 12 c of the access point12 and the second communication control section 13 c of the terminalcooperatively control the aforementioned per-group communication. Thus,a configuration in which the first communication control section 12 cand the second communication control section 13 c are combined togethercan also be simply taken as a communication control section.

FIG. 4 is a diagram for explaining the operation of the group creationsection according to the first embodiment. Note that, in the descriptionof FIG. 4, the operation of the first group creation section 12 g isdescribed as the operation of the access point 12 and the operation ofthe second group creation section 13 g is described as the operation ofthe terminal. In FIG. 4, the access point 12 transmits, in turn, aresponse request packet to all the terminals 131 to 133 and 141 to 142(not shown). The access point 12 can transmit a response request packetto the terminals in any order; here, a response request packet istransmitted to the terminals 131, 132, 133, 141, and 142 in this order.

Once the terminal 131 has received the response request packet from theaccess point 12, the terminal 131 responds to the access point 12 with aresponse packet P131 having set therein specific information (A131)about the terminal 131. The specific information (A131) about theterminal 131 includes, for example, an IP address and a MAC address setin the terminal 131. The response packet P131 is also received by otherterminals within the reach of the radio wave. Specifically, the responsepacket P131 can also be received by the terminals 132 and 133. However,the response packet P131 cannot be received by the terminals 141 and142.

Once the terminal 132 has received the response request packet from theaccess point 12, the terminal 132 responds to the access point 12 with aresponse packet P132 having set therein specific information (A132)about the terminal 132 and the specific information (A131) having beenreceived so far. The response packet P132 can also be received by theterminals 131 and 133. Similarly, once the terminal 133 has received theresponse request packet from the access point 12, the terminal 133responds to the access point 12 with a response packet P133 having settherein specific information (A133) about the terminal 133 and thespecific information (A132, A131) having been received so far. That is,in the response packet P133, is set the specific information (A131 toA133) about all the terminals 131 to 133 which can receive from eachother radio waves transmitted therefrom.

Once the terminal 141 has received the response request packet from theaccess point 12, the terminal 141 responds to the access point 12 with aresponse packet P141 having set therein specific information (B141)about the terminal 141 and specific information having been received sofar. Note that the terminal 141 does not receive the response packetsP131 to P133 because of the presence of the radio-wave shield 15.Therefore, in the response packet P141, is set only the specificinformation (B141) about the terminal 141. Similarly, the terminal 142responds to the access point 12 with a response packet P142 having settherein specific information (B142) about the terminal 142 and thespecific information (B141) having been received so far. That is, in theresponse packet P142, is set the specific information (B141 to B142)about all the terminals 141 to 142 which can receive from each otherradio waves transmitted therefrom.

The access point 12 can identify which terminals can receive from eachother radio waves transmitted therefrom, based on the packets P131 toP133 and P141 to P142 responded from the terminals 131 to 133 and 141 to142. The access point 12 then creates a plurality of groups by groupingtogether terminals which can receive from each other radio wavestransmitted therefrom. Specifically, the access point 12 groups theterminals 131 to 134 as group A based on the specific information set inthe packet P133, and the terminals 141 to 142 as group B based on thespecific information set in the packet P142.

FIG. 5 is a diagram for explaining the operation of the communicationcontrol section according to the first embodiment. Note that, in thedescription of FIG. 5, the operation of the first communication controlsection 12 c is described as the operation of the access point 12 andthe operation of the second communication control section 13 c isdescribed as the operation of the terminal. In FIG. 5, as a method ofswitching communication between the groups A and B, there is shown amethod using RTS/CTS packets. Referring to FIG. 5, it is assumed that,after the terminals have been grouped, data to be transmitted isgenerated from one of the terminals belonging to the group A. In thiscase, in order to seek a transmission permission to the group A, the oneof the terminals belonging to the group A transmits an RTS packet 221 tothe access point 12. Once the access point 12 has received the RTSpacket 221, the access point 12 responds with a CTS packet 222 as atransmission permission to the group A. In the CTS packet 222, is setspecific information (A131 to 133) of all the terminals belonging to thegroup A and the length of a communication interval assigned to the groupA. Note that the length of a communication interval refers to a periodof time during which communication is permitted (a communicationpermitted time).

The CTS packet 222 is received by all the terminals 131 to 133 and 141to 142 belonging to the groups A and B. The terminals 131 to 133 and 141to 142 each determine whether or not the communication interval isassigned to a group to which the terminal belongs, based on whether ornot specific information about the terminal is set in the received CTSpacket 222. Since the specific information about the terminals 131 to133 are set in the received CTS packet 222, the terminals 131 to 132belonging to the group A determine that the communication interval isassigned to the group A. If it is determined to be in a communicationinterval, the terminals 131 to 133 belonging to the group A perform datacommunication with the access point 12 according to the CSMA/CA schemeuntil the communication interval ends.

The terminals 141 to 142 belonging to the group B, on the other hand,determine that the group B is in a standby interval since the specificinformation about the terminals 141 to 142 are not set in the receivedCTS packet 222. If it is determined to be in a standby interval, theterminals 141 to 142 belonging to the group B stand by for datacommunication with the access point 12 until the standby interval ends.

Now, the case is considered where data to be transmitted is generatedfrom one of the terminals belonging to the group B during the standbyinterval of the group B. In this case, the one of the terminalsbelonging to the group B transmits an RTS packet 223 to the access point12 after the standby interval ends. Once the access point 12 hasreceived the RTS packet 223 from the terminal belonging to the group B,the access point 12 responds with a CTS packet 224 as a transmissionpermission to the group B. The terminals 141 to 142 belonging to thegroup B determine, by the reception of the CTS packet, that theterminals 141 to 142 are in a communication interval. On the other hand,the terminals 131 to 133 belonging to the group A determine, by thereception of the CTS packet 224, that the terminals 131 to 133 are in astandby interval. The wireless access system according to the firstembodiment controls data communication between the access point 12 andthe terminals 131 to 133 and 141 to 142 by repeating the aforementionedoperation.

As described above, according to the wireless access system of the firstembodiment, the group creation section divides a plurality of terminalsinto groups so as to create groups of terminals, each group havingterminals which can receive from each other radio waves transmittedtherefrom. Then, the communication control section assigns acommunication interval and a standby interval individually to eachgroup, and controls communication between the access point 12 and theterminals 131 to 133 and 141 to 142 to be performed on a per-groupbasis. By doing so, the wireless access system according to the firstembodiment can prevent the occurrence of the hidden terminal problembetween terminals which cannot receive from each other radio wavestransmitted therefrom. Also, since RTS/CTS packets are transmitted andreceived on a per-group basis, the time consumed by overhead is reducedas compared with a method of transmitting and receiving RTS/CTS packetson a per-terminal basis, which is one of the conventional methods ofsolving the hidden terminal problem; accordingly, the reduction inthroughput can be minimized.

Note that in the wireless access system according to the firstembodiment the access point 12 may create, with the use of a methodother than the aforementioned operation, a group where terminals whichcan receive from each other radio waves transmitted therefrom aregrouped together. For example, a user can identify which terminals canreceive from each other radio waves transmitted therefrom, in view ofthe radio-wave shield 15, the distance between terminals, or the like.The user instructs the access point 12 using such information. Theaccess point 12 may group together terminals which can receive from eachother radio waves transmitted therefrom, in accordance with theinstruction from the user.

The access point 12 may transmit group information to all the terminals131 to 133 and 141 to 142 at once after the grouping of the terminals iscomplete. In the group information, is set information indicating thegroup A, the group B, or the like. Each of the terminals 131 to 133 and141 to 142 can identify, by the reception of the group information,which group the terminal belongs to. Therefore, when transmitting acommunication permission to a terminal, the access point 12 can usegroup information instead of specific information about the terminal.

If the access point 12 has no ongoing data communication with terminalsduring a communication interval of a group over a certain period oftime, the access point 12 may transmit a CTS packet to the terminals toforcefully terminate the communication interval of the group. By this,the wireless access system reduces a wasted assignment of acommunication interval, and accordingly, efficient communication can beperformed.

Although the access point 12 determines the communication interval andthe standby interval to be assigned to each group, according to thenumber of terminals belonging to each group, such a determination may bemade in accordance with the instruction from an operator. By this, thewireless access system can assign a communication interval and a standbyinterval to each group in compliance with the intention of the operator,making it possible to provide a more flexible system operation.

Furthermore, the access point 12 may determine the communicationinterval and the standby interval to be assigned to each group,according to a predetermined timing. In this case, the access point 12notifies, after the grouping of terminals is complete, all the terminals131 to 133 and 141 to 142 about a timing at which switching between thecommunication interval and the standby interval is performed. Forexample, in the case where the access point 12 alternately assigns thegroup A with a five-second communication permitted time and the group Bwith a three-second communication permitted time, the access point 12notifies the terminals 131 to 133 and 141 to 142 about suchcommunication permitted times. The terminals 131 to 133 and 141 to 142switch between the communication interval and the standby intervalaccording to the notified timings.

Second Embodiment

A wireless access system according to a second embodiment is differentfrom that of the first embodiment in the operation of a communicationcontrol section. The communication control section according to thefirst embodiment determines the communication interval and the standbyinterval to be assigned to each group, according to the number ofterminals belonging to each group. On the other hand, the communicationcontrol section according to the second embodiment determines thecommunication interval and the standby interval to be assigned to eachgroup, according to the amount of transmission standby data held interminals belonging to each group. Note that the operations of sectionsother than the communication control section are the same as those ofthe first embodiment.

FIG. 6 is a diagram for explaining the operation of the communicationcontrol section according to the second embodiment. Note that, in thedescription of FIG. 6 too, the operation of a first communicationcontrol section 12 c is described as the operation of an access point 12and the operation of a second communication control section 13 c isdescribed as the operation of a terminal. In FIG. 6, the access point 12transmits, after the grouping of terminals 131 to 133 and 141 to 142 iscomplete, a transmission-standby information request packet 225 to allthe terminals so as to know the actual amount of transmission standbydata. The terminals 131 to 133 and 141 to 142 having received thetransmission-standby information request packet 225 responds, if thereis transmission standby data, to the access point 12 with atransmission-standby information packet 226 having set therein thelength of transmission standby data.

The access point 12 determines the order and length of a communicationinterval to be assigned to groups A and B based on the length oftransmission standby data set in the transmission-standby informationpacket 226. For example, assume that the amount of transmission standbydata the access point 12 has actually received is such that the group Ahas 2 Kbyets of data in total and the group B has 1 Kbyet of data intotal. In this case, the access point 12 can assign the group A having agreater amount of data with a two-second communication interval first,and then assign the group B with a one-second communication interval.The access point 12 sets in a CTS packet 227 specific information aboutterminals to which communication is permitted and the length of thecommunication interval, and transmits the CTS packet 227 to theterminals 131 to 133 and 141 to 142.

The terminals 131 to 133 and 141 to 142 determine that the terminals 131to 133 and 141 to 142 are in a communication interval or a standbyinterval, based on the received CTS packet 227, and performs datacommunication with the access point 12. The operation of the terminals131 to 133 and 141 to 142 after receiving the CTS packet 227 is the sameas that of the first embodiment. Once a sequence of data communicationhas completed between the access point 12 and the terminals 131 to 133and 141 to 142, the access point 12, once again, transmits atransmission-standby information request packet 228 to the terminals 131to 133 and 141 to 142. By repeating the aforementioned operation, thewireless access system can prevent the occurrence of the hidden terminalproblem between terminals which cannot receive from each other radiowaves transmitted therefrom.

As described above, according to the wireless access system of thesecond embodiment of the present invention, the communication intervaland the standby interval to be assigned to each group are determinedaccording to the amount of transmission standby data held in terminalsbelonging to each group. Therefore, the wireless access system canassign the transmission interval and the standby interval according tothe actual amount of traffic, and accordingly, communication efficiencycan be increased.

In the wireless access system according to the second embodiment, theterminals 131 to 133 and 141 to 142 may transmit the length of datagenerated during a preceding standby interval (namely, transmissionstandby data), during the following communication interval. FIG. 7 is adiagram for explaining the operation of a wireless access system whichtransmits, during a communication interval, the length of data generatedduring a preceding standby interval. With reference to FIG. 7, the caseis considered where, for example, data to be transmitted is generatedfrom one of the terminals belonging to group B during a standbyinterval. In this case, the one of the terminals belonging to the groupB transmits to the access point 12 a transmission-standby informationpacket 228 in which the length of the transmission data generated duringthe standby interval is set.

The access point 12 can determine the order and length of acommunication interval to be assigned to the groups A and B based on thereceived transmission-standby information packet 228. This eliminatesthe need for the access point 12 to inquire, using thetransmission-standby information request packet 225, about thetransmission standby data generated during the standby interval, andtherefore the access point 12 can perform efficient communication inwhich the reduction in throughput is minimized.

Third Embodiment

FIG. 8 is a diagram showing an exemplary network configuration of awireless access system according to a third embodiment. The wirelessaccess system according to the third embodiment includes an access point20 having a plurality of remote antennas, and thus can cover a widecommunication area. In FIG. 8, the wireless access system according tothe third embodiment includes the access point 20, a remote antenna 21,a remote antenna 22, a transmission line 23, and a plurality ofterminals 131 to 133 and 141 to 142. The access point 20 performscommunication with the terminals 131 to 133 via the remote antenna 21,and with the terminals 141 to 142 via the remote antenna 22. Note thatthe transmission line 23 may be free space which is connected by a radiosignal.

Here, it is assumed that the terminals 131 to 133 cannot receive radiowaves transmitted from the terminals 141 to 142 for the reason that theremote antenna 21 and the remote antenna 22 are located far away fromeach other, or the like. It is also assumed that the terminals 141 to142 cannot receive radio waves transmitted from the terminals 131 to132. In such a case, as with the wireless access system according to thefirst embodiment, the wireless access system according to the thirdembodiment groups the terminals 131 to 133 as group A and the terminals141 to 142 as group B, and then assigns a communication interval and astandby interval to each group. By doing so, the wireless access systemaccording to the third embodiment can prevent the occurrence of thehidden terminal problem between terminals which cannot receive from eachother radio waves transmitted therefrom.

As described above, according to the wireless access system of the thirdembodiment, as with the wireless access systems of the first and secondembodiments, the occurrence of the hidden terminal problem can beprevented. Further, by providing a plurality of remote antennas to asingle access point 20, a wider communication area can be covered.

Fourth Embodiment

FIG. 9 is a diagram showing an exemplary network configuration of awireless access system according to a fourth embodiment. The wirelessaccess system according to the fourth embodiment is such that thecommunication between the access point 20 and the remote antennas 21 and22 in the wireless access system according to the third embodiment isreplaced with communication using an optical signal. In FIG. 9, thewireless access system according to the fourth embodiment includes anaccess point 20, an electrical-optical/optical-electrical conversionsection 24, a remote antenna unit 25, a remote antenna unit 26, anoptical transmission line 27, and a plurality of terminals 131 to 133and 141 to 142.

The optical-electrical/electrical-optical conversion section 24 convertsan electrical signal inputted from the access point 20 into an opticalsignal, and also converts an optical signal inputted from the opticaltransmission line 27 into an electrical signal and outputs theelectrical signal to the access point 20. The optical transmission line27 connects between the electrical-optical/optical-electrical conversionsection 24 and the remote antenna units 25 and 26. For the opticaltransmission line 27, an optical fiber cable is used, for example. Theremote antenna units 25 and 26 each convert an optical signal inputtedfrom the optical transmission line 27 into a radio signal and transmitthe radio signal to free space, and also convert a radio signal receivedfrom the terminals 131 to 133 and 141 to 142 into an optical signal andtransmit the optical signal to the optical transmission line 27. Theremote antenna unit 25 is composed of an antenna 21 and anoptical-electrical/electrical-optical conversion section 28. The remoteantenna unit 26 is composed of an antenna 22 and anoptical-electrical/electrical-optical conversion section 29.

As with the wireless access system according to the third embodiment,the wireless access system according to the fourth embodiment groups theterminals 131 to 133 as group A, and the terminals 141 to 142 as groupB, and then assigns a communication interval and a standby interval toeach group. By doing so, the wireless access system according to thefourth embodiment can prevent the occurrence of the hidden terminalproblem between terminals which cannot receive from each other radiowaves transmitted therefrom.

As described above, according to the wireless access system of thefourth embodiment, as with the wireless access system of the thirdembodiment, the occurrence of the hidden terminal problem can beprevented. Further, since a plurality of antenna units can be installedin a wide range using the optical transmission line 27, it is possibleto expand the communication area which can be covered by a single accesspoint 20.

Note that the wireless access system according to the fourth embodimentmay be configured such that a plurality of access points are providedand the plurality of access points are switched from one to anotherusing a switch. FIG. 10 is a diagram showing an exemplary networkconfiguration of a wireless access system which performs switchingbetween a plurality of access points. In FIG. 10, the wireless accesssystem switches between an access point 31 and an access point 32 usinga switch 33. This allows the terminals 131 to 133 and 141 to 142 tocommunicate with any access point. The wireless access system switchesbetween the access points 31 and 32 based on the difference in thecommunication method of the terminals, for example. By this, even in thecase where terminals using different communication methods are present,the wireless access system can prevent the occurrence of the hiddenterminal problem.

INDUSTRIAL APPLICABILITY

The wireless access system of the present invention is useful as amethod of solving the hidden terminal problem which occurs, for example,in a wireless LAN system, or the like.

1. A wireless access system for performing transmission and reception ofdata using a radio wave, the wireless access system comprising: aplurality of terminals each performing transmission and reception of aradio wave in accordance with a predetermined instruction; and an accesspoint which divides the plurality of terminals into groups so as tocreate a plurality of groups of terminals, and provides to the pluralityof terminals the predetermined instruction to give a communicationpermission, the plurality of groups each having terminals which canreceive from each other radio waves transmitted therefrom, thecommunication permission allowing only one of the plurality of groups tocommunicate with the access point.
 2. The wireless access systemaccording to claim 1, further comprising: at least one remote antennawhich performs, in a location away from the access point, transmissionand reception of a radio wave with any of the plurality of terminals;and a transmission line which connects between the remote antenna andthe access point.
 3. The wireless access system according to claim 2,wherein the transmission line is an optical transmission line throughwhich an optical signal is transmitted, and the wireless access systemfurther comprises: an electrical-optical/optical-electrical conversionsection for converting an electrical signal inputted via the accesspoint into an optical signal and outputting the optical signal to theoptical transmission line, and for converting an optical signal inputtedvia the optical transmission line into an electrical signal andoutputting the electrical signal to the access point; and anoptical-electrical/electrical-optical conversion section for convertingan optical signal inputted via the optical transmission line into anelectrical signal and outputting the electrical signal to the remoteantenna, and for converting an electrical signal inputted via the remoteantenna into an optical signal and outputting the optical signal to theoptical transmission line.
 4. The wireless access system according toclaim 1, wherein the access point provides, as the predeterminedinstruction, to the plurality of terminals specific information aboutand a communication permitted time for terminals which are included in agroup to which the communication permission is given.
 5. The wirelessaccess system according to claim 1, wherein the access point includes: afirst group creation section for transmitting a response request packetto each of the plurality of terminals in any order, receiving, as aresponse to the response request packet, a response packet from each ofthe terminals, and then creating the plurality of groups based onspecific information set in each of the received response packets, andthe terminals each include: a second group creation section for setting,in the response packet, specific information about its own terminal andspecific information received from other terminals, and responding tothe access point with the response packet as a response to the responserequest packet received from the access point.
 6. The wireless accesssystem according to claim 5, wherein the access point further includes:a first communication control section for setting, in a case where theaccess point receives from any of the plurality of terminals an RTSpacket as a transmission request, specific information about and acommunication permitted time for terminals which are included in a groupfrom which the RTS packet has been transmitted, in a CTS packet as aninstruction to give the communication permission, and transmitting theCTS packet to the plurality of terminals, and the terminals each furtherinclude: a second communication control section for transmitting, in acase where data to be transmitted is generated, the RTS packet to theaccess point, receiving from the access point the CTS packet as aresponse to the RTS packet, and determining, if the specific informationset in the received CTS packet contains specific information about itsown terminal, that the communication permission is given to its ownterminal, and performing communication with the access point for aperiod of the communication permitted time.
 7. The wireless accesssystem according to claim 4, wherein the access point sets apredetermined time for the communication permitted time.
 8. The wirelessaccess system according to claim 4, wherein the access point determinesthe communication permitted time in accordance with an instruction froman operator.
 9. The wireless access system according to claim 4, whereinthe access point determines the communication permitted time accordingto a number of terminals included in the plurality of groups.
 10. Thewireless access system according to claim 4, wherein the access pointdetermines the communication permitted time according to an amount oftransmission standby data held in the terminals included in theplurality of groups.
 11. The wireless access system according to claim2, wherein the transmission line is used for free space.
 12. Thewireless access system according to claim 3, wherein the opticaltransmission line is an optical fiber cable.
 13. A wireless accessmethod of performing transmission and reception of data using a radiowave, the wireless access method comprising: a plurality of terminalseach performing transmission and reception of a radio wave in accordancewith a predetermined instruction; and an access point which divides theplurality of terminals into groups so as to create a plurality of groupsof terminals, and provides to the plurality of terminals thepredetermined instruction to give a communication permission, theplurality of groups each having terminals which can receive from eachother radio waves transmitted therefrom, the communication permissionallowing only one of the plurality of groups to communicate with theaccess point, wherein the access point comprises the steps of:transmitting a response request packet to the plurality of terminals inany order; receiving, as a response to the response request packet, aresponse packet from each of the terminals; and creating the pluralityof groups based on specific information set in each of the receivedresponse packets, and the terminals each comprise the steps of:receiving the response request packet from the access point; andsetting, in the response packet, specific information about its ownterminal and specific information received from other terminals, andresponding to the access point with the response packet as a response tothe response request packet.
 14. The wireless access method according toclaim 13, wherein the access point further comprises the steps of:setting, in a case where the access point receives from any of theplurality of terminals an RTS packet as a transmission request, specificinformation about and a communication permitted time for terminals whichare included in a group from which the RTS packet has been transmitted,in a CTS packet as an instruction to give the communication permission;and transmitting the CTS packet to the plurality of terminals, and theterminals each further comprise the steps of: transmitting, in a casewhere data to be transmitted is generated, the RTS packet to the accesspoint; receiving from the access point the CTS packet as a response tothe RTS packet; and determining, if the specific information set in thereceived CTS packet contains specific information about its ownterminal, that the communication permission is given to its ownterminal, and performing communication with the access point for aperiod of the communication permitted time.